How can I tell if my PSU is insufficent

Iwf

Iwf

Associate
Joined
26 Sep 2020
Posts
58
I've 7800x3d, 4090 watercooled solution with a 750 watt power supply. Other than work I mainly use for MS Flight Sim.

Recent testing using 3dmark bench marking and stability tests suggest it's both stable, producing 'excellent' scores with CPU and GPU temps sub 60c. MSI after burner is used to spice things up a bit,. but no other overclocking has been attempted I side loaded a 450 PSU tiggered from the primary so in theory I have 1000w of power available. A power meter records a max of 628 watts on load. But i'd prefer to run only one PSU.

But everything I read suggests 750w isn't sufficient. The exam question is thus how can tell if the system is being throttled. What data should I be looking at?

thanks
 

Iwf

Iwf

Associate
OP
Joined
26 Sep 2020
Posts
58
If the psu is insufficient you will get shutdowns, black screens and general instability. Personally I would not be running a 4090 off a 750w psu. What make and model is it?
Thanks

It's a Corsair SF750. Never had any problems with it and none of the symptoms you speak of. When I ran the two PSU, I used the 450 to run the PC and the 750 for the 4090, but as I say overall it never seem to draw more than 650w. It's of course possible the Corsair are built well
 
Soldato
Joined
15 Oct 2019
Posts
12,024
Location
Uk
Id only be concerned if you were running a 4090 with a 13900K/14900k or were heavily overclocking the 4090

The 7800X3D is very efficient and won’t be pulling more than 100w when gaming and despite the 4090 being rated at 450w it very rarely pulls that much.
 
Last edited:
Soldato
Joined
14 Aug 2018
Posts
3,507
While looking at the nominal power usage it should be fine but what I've seen can trip PSU's up is transient spikes. I'm not sure how the 4090 fares with this but I suspect it is one of the reasons a 750w PSU is considered as not quite enough.

I do remember that PSU's are supposed to be at their most efficient when using around half their rated power. Whether this plays into what is recommended I'm not sure.
 
Soldato
Joined
23 Jun 2004
Posts
4,734
Location
Blackburn
You'll find out if you have issues when you remove the 450w. Your power meter might only be recording ~630w but you'll likely also be having transient spikes which could cause black screens and crashes if the 2nd 450w removed even if they are just for a second.
 
Permabanned
Joined
30 Dec 2021
Posts
3,642
Location
Yorkshire
obviously you going to be under volting your GPU because that's the thing to do...
in that case i would think an SF750 would be fine, i ran a 5800x and a 3090 on an SF750 without any problems.

what CPU are you running

is your case full of RGB and SSD's/HDD's?
do you have loads of USB stuff plugged in?
 
Last edited:
Soldato
Joined
12 Sep 2003
Posts
10,263
Location
Newcastle, UK
Recommended PSU wattage here - https://www.corsair.com/uk/en/explorer/diy-builder/power-supply-units/what-psu-is-best-for-rtx-4090/

If you'd prefer to run just one PSU then simply spend the cash and buy a 1000W unit and be happy for the next 10 years or so.

And just a reminder that PSU's are most efficient when not fully loaded, like @MartinPrince has mentioned, so even though it may run on a 750W PSU I'd imagine it wouldn't be happy about it. Longevity etc.

If you've bought a very nice and expensive 4090 why skimp on the PSU. Do the right thing and power the rig correctly.
 
Soldato
Joined
7 Jul 2004
Posts
7,806
Location
Gloucester
I have an Corsair SF750 running my 4090 with a 13900k
No issues with it at all

Reviews show the SF750 going over 900w before it has problems.
I do have a white 850w Asus Loki but I haven't got around to swapping them over yet. I got it to because it's white so it'll match my build
 
Associate
Joined
5 Mar 2017
Posts
2,278
Location
Cambridge
As you are not experiencing any issues, is fine. As the PSU gets older, you may, or may not, but the only real negative I would think of is (as it’s working without issues) minimal margin for spikes.
When analysing the efficiency curve of a PSU, af the low and at the top end the PSU won’t be as efficient. Working close to it’s maximum capacity could be detrimental to its longevity, but considering it’s a good quality PSU, that shouldn’t be an issue.
As mentioned before, some SFX PSUs are designed to work in tougher conditions, and I would t be surprised if that PSU could supply at least 150W on top of the rated output.
I ended up going overkill and getting a 1300W because price was good, space isn’t an issue for me and the 7800x3D + 4080 are well below the PSU’s capacity and, as result, only during heatwave or very rare occasions the fan kick in.
 
Associate
Joined
17 May 2009
Posts
733
Location
Kirkwall, Orkney
As the guys say, unless you experience random crashes or shutdowns you should be ok. I do believe most 4090's come with warnings that you should use a minimum of 850W power supply (my 7900XTX did as well in instruction manual) but I believe this will rule in the factor you may use a poorer standard of PSU which your Corsair model is not. I have used the the 1000W supply for my last two builds and can highly recommend the 2023 model I have.
 
Last edited:
Back
Top Bottom